1. Field of the Invention
This invention relates to noise control devices and more particularly to a noise control device which is associated with a sound insulating wall or the like to significantly improve its sound reducing effect alleviating noise emitted from a noise source such as a railroad, highway or the like on which an electric car or automobiles operate.
2. Description of the Prior Art
Sound insulating walls and the like have been developed for the purpose of alleviating a public nuisance caused by various kinds of noises. In order to alleviate such noise, various measures for counteracting the effects of noise sources have been attempted. Such measures, however, have limits to their performance and are often difficult to implement.
For reducing noise, it has been the common practice to provide a barrier such as a sound insulating wall or the like and position it between a noise source and a noise receiving region for the purpose of intercepting propagation of the noise or to provide a barrier constructed to completely surround a noise source and shield it.
However, it is a matter of course that the former barrier is limited in its sound insulating effect, while the latter barrier requires other means such as a heat dissipation device, a ventilation device or the like and hence such barriers are complex in construction and difficult to. For insulating noise produced from vehicles such as a high speed electric car running on an elevated railroad or the like, use has been made of a sound insulating wall. That part of the noise which passes over the upper edge of the sound insulating wall and which is refracted in a sound shadow behind the sound insulating wall causes the sound insulating wall to reduce the noise by at most 20 dB. Particularly, a sound insulating wall provided at a position where one can see a noise source has substantially no effect.
In order to eliminate such disadvantages, attempts have been made to provide a shelter wall arranged along a railroad and surrounding the overall length thereof. But, such a shelter wall prevents houses adjacent thereto from being exposed to the sun, is expensive, requires a ventilation device, interrupts passenger's visual field, etc. and hence is difficult to use operationally.
The above mentioned problem has also been encountered with means for insulating noise produced from automobiles and from power driven units provided for industry.